Atmospheric diffusers have become a widely accepted technology in the paper pulp treatment art, both for washing and bleaching. While atmospheric diffusers have a wide variety of advantages over other types of washing and bleaching devices, their performance is still not optimum despite continuous development work over the past 25 years. A major aspect of conventional diffusers that could desirably be improved is efficiency, conventional bleaching diffusers having relatively high chemical consumption.
Perhaps the basic reason for any shortcomings in efficiency that do exist in conventional diffusers is that the diffuser does not operate as a perfect displacement machine, but rather only acts as a combination of a mixer and thickener, and only partially is a displacement machine. Ideally, the diffuser operation would consist of a series of individual batch operations, each one being a complete cycle, such that during the diffuser upstroke the pulp and the screen assembly travel at the same speed with relation to each other and are therefore--relatively speaking--stationary. Unfortunately, that ideal does not translate into practice, but rather in actual diffusers the diffuser ends up moving significantly faster than the pulp flowing upwardly in the tower.
It has been determined that during normal operation of a diffuser, the static pressure as measured underneath the diffuser is lower than it would be at the same elevation in the tower without the diffuser. What appears to happen is that the pulp is held between the annular screen assemblies by the pressure differential across the screen. Thus the screen assembly acts like a piston which supports some of the weight of the pulp column above. This results in a region of lower static pressure under the diffuser, and therefore it is easier for some of the liquid from the treatment liquid introduction nozzles to move into the lower region, creating more dilute zones. Consequently, not all of the wash or treatment liquid from the nozzles goes directly across to the screens to displace the dirty liquid, but some of it first gets mixed with the incoming unwashed or untreated pulp. This is one of the major contributing factors to the lower efficiency than is desirable problem mentioned above.
Capacity in existing systems is also somewhat limited by the manner in which the downstroke is initiated. Typically, there is a 3-8 second wait (out of a total stroke time of about 50 seconds) at the top of the stroke when extraction does not take place, in order for the internal and external pressures associated with the screen to equalize, before the downstroke is initiated. For maximum capacity the downstroke should be initiated immediately when the pressure equalizes, rather than waiting a predetermined time period; utilizing a transducer sensing pressure within a screen, it would be possible to know immediately when pressure was equalized, and start the downstroke then. According to the invention, efficiency is enhanced by taking steps to ensure that the upstroke speed of the screen will be only very slightly faster than the pulp. This is accomplished by determining the pressure differential under the diffuser and comparing it to the normal static pressure at that elevation, and then controlling the speed of the upstroke so that the pressure under the diffuser is maintained only slightly below the corresponding static pressure.
According to the invention it is also desirable to enhance the ability to provide an early warning indication of a tendency of the screens to plug. This is accomplished by providing a more sensitive pressure sensor within the hollow interior of the screen, prior to the orifice thereof.
It has also been recognized that sometimes after completion of the downstroke, the diffuser screen has a tendency to jump back up, without assistance from the hydraulic system. The pressure causing the screen to "jump" can be greater than that generated by the hydraulic pump which supplies fluid to the hydraulic system for normally moving the screen assemblies. This "jumping" tendency is typically caused by the presence of carbon dioxide gas generated in the tower, and entrained air in the incoming pulp. Those gases are compressed during the rapid downstroke of the screen assembly, and once the downstroke force has been terminated, the compressed gas expands, pushing the diffuser up faster that it is being driven by the hydraulic system itself. Thus the diffuser at the beginning of the upward stroke does not move at the proper speed in relation to the pulp, increasing the tendency to plug and decreasing efficiency.
According to the invention, the tendency of the diffuser screen assembly to "jump" is arrested by initially causing the cylinder to cause the screen assembly to move upwardly in a sequence of controlled small increments during the upstroke. A simple pressure transducer is provided in the hydraulic line going into the cylinder for the upstroke, and when the pressure transducer indicates that it is the hydraulic oil that is lifting the diffuser up not the pulp, at which point the upward movement then is caused to be smooth.
It is the primary object of the present invention to provide enhanced efficiency for an atmospheric diffuser, and a method of treating the suspension, in the pulp and paper art. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.